Various processes are known for the determination of physical and biochemical parameters of particles, such as microorganisms or cells, in electrolytes. These electrolytes experience a change in resistance induced by a particle in a measurement pore as a result of the displacement of the electrolytes. (Thom, R., et al., "The electronic volume determination of blood solids and sources of errors therein;" Z. ges. exp. Med. 151,331-349/1969). Accordingly, prior art devices typically perform amplitude measurement according to Coulter, wherein only the maximum amplitude of a pulse is used, as well as pulse width, in which the pulse width is evaluated as a measure of the particle volume.
In both processes, measurement pores of monocrystalline structures are used, e.g., watch jewels, which for reasons of stability and measurement technology have a length to diameter ratio of virtually one to one. In measurement pores for small particles, the pore length must, for the stability reasons mentioned, be at least equal to the diameter, since otherwise with the conical opening, the remaining wall thickness, which corresponds to the capillary length, becomes unstable. In measurement pores for large particles, i.e., from 45 .mu.m, the pore length must be equal to or greater than the diameter of the particle so that the particle is completely taken in by the pore during measurement to prevent measurement errors.
In the prior art devices the measurement pore is disposed between two electrodes whose distance from the measurement pore is a multiple of the length of the measurement pore (German Pat. No. 21 45 531 C2). This distance is up to 100 times the length of the measurement pore.
From German Pat. No. 36 26 600 A1, a device for determining the properties of particles suspended in liquids is known. This device has, as a measurement pore, a channel which runs through a material produced by using multilayer bonding technology. This results in a plurality of layers of material which are inseparably bonded to each other. Thus, it is possible to produce the channel with a gradually altered cross section.
The disadvantage of the process using the prior art measurement pores is that, for stability reasons, capillary lengths of less than 40 .mu.m cannot be stably produced with the known techniques for processing monocrystalline structures and are mechanically unstable in use. At the lower measurement limit, the limit is a function of the ratio of the displaced volume to the entire volume inside the measurement pore. When there is a large volume of the measurement pore along with a small particle volume, the relative change in resistance is low and delivers no usable measurement results. Consequently, only large particles are measurable.
It is known that there must always be only one particle inside the measurement pore, since otherwise there would be erroneous measurements relative to the particle number and size. Due to the large volume of the measurement pore, that is not completely guaranteed, i.e., the coincidence limit is relatively low. Consequently, with high particle concentrations, high rates of dilution before the measurement are essential.
Since the electrodes may only be placed at the aforementioned large distance from the measurement pore, a nonhomogeneous electrical field is produced in front of the pore. As a result, large particles cause a change in electrical resistance at a long distance in front of the pore and consequently broaden of the pulse. This then causes errors in the pulse surface analysis and additional coincidence errors.
A further disadvantage is that the measurement process cannot be performed on-line. This is because there is a high sensitivity to external electromagnet radiations, since one side of the measurement pore may be at ground potential, whereas the other side is linked to the input of the amplifier which acts as an antenna.
Moreover, the measurement signal is a function of the volume, only not of the shape of the particles. However, the shape is of great interest in various applications, e.g., in biological measurements of cells.
In the prior art measurement processes, there is a cubic relationship between diameters of the particles and the amplitude of the measurement pulse, which results in a very small dynamic range of the measurement process. Thus, for example, with a desired resolution of 0.05 .mu.m (diameter of the particle) at a lower measurement limit of 1 .mu.m, there is a maximum dynamic range of 1:3.5 with an 8-bit resolution of the measurement signal.
Another disadvantage is due to the high internal resistance of the capillary. The noise voltage of the measurement arrangement is increased by the high internal resistance, which is proportional to the root of the internal resistance. This noise voltage also restricts the shifting of the measurement limit in the direction of smaller particles.
In the aforementioned processes, only simple resistance measurements are performed which produce information concerning only the physical parameters. If information is also needed with regard to the biochemical behavior of microorganisms or cells in their interior or on their surface, these investigations must be carried out using expensive preparation techniques (micromanipulators or microelectrodes, patch-clamp technique) as well as simple resistance measurements.